1. Technical Field to which the Invention Belongs
The present invention relates to an optical scanner and an image formation apparatus including the optical scanner.
2. Prior Art
Conventionally, optical scanners have been used for image formation apparatuses such as laser beam printers, laser facsimile machines and digital copy machines. Known as an optical scanner of this kind is an apparatus which includes a semiconductor laser serving as a light source, a polygon mirror (a rotating polygon mirror), a first image formation optical system for making a ray bundle from the semiconductor laser form a line image on the polygon mirror, a second image formation optical system for forming an image of uniform spots at a uniform velocity on a scanning plane, a scanning start signal detector for detecting a ray bundle scanned by the polygon mirror, and a detection optical system for gathering ray bundles from the semiconductor laser to the scanning start signal detector (see, e.g., Japanese Laid-Open Publication No. 2001-166239.)
In order to enhance the speed of image formation and achieve higher resolution, techniques for increasing scan speed have been developed. Proposed as one of those techniques is a so-called overfilled polygon optical scanner in which a light beam whose width is wider than the plane width of the individual deflecting planes of the polygon mirror is applied to the polygon mirror.
In an overfilled optical scanner, the deflecting planes of the polygon mirror cut off and reflect a portion of the incoming light beam and scan the reflected light. However, the portion of the light beam cut off by the deflecting planes varies depending upon the scan angle. Thus, the distribution of luminous energy in the scan line is significantly affected by the distribution of the intensity of the light beam.
When an optical scanner is designed, it is generally assumed that the beam profile (i.e., the energy distribution) of a light beam produced from the light source device is a Gaussian distribution, which is symmetrical about the peak of the beam intensity. Nevertheless, in the actual optical scanners, the beam profile is often not symmetrical about the peak due to errors caused in the fabrication of the light source device. Moreover, in some of the actual optical scanners, a displacement of the axis of the produced light occurs. To address this, an optical scanner aimed at achieving an approximately uniform luminous-energy distribution on a scanning plane has been proposed (see Japanese Laid-Open Publication No. 9-96769, which will be hereinafter referred to as a “patent reference”).
In the optical scanner disclosed in the patent reference, powers of the light beam are detected in multiple positions, which correspond to a scanning plane to be scanned, and the optical axis is adjusted so that the ratio of any one of the detected powers to another is approximately 1 to 1.
In order to increase scan speed further, an overfilled optical scanner in which a plurality of light beams are used has been proposed. For example, as shown in FIG. 17A, a scanner, in which a light source 100 produces a first light beam L1 and a second light beam L2 and these two light beams L1 and L2 are directed to a polygon mirror, has been proposed.
However, in such a scanner in which a plurality of light beams are used, it is difficult to make the distribution of luminous energy uniform by using the above-described prior technique alone. More specifically, the above-described prior technique is intended to make a single light beam have a uniform luminous energy distribution. In other words, the prior technique makes the beam profile symmetrical as much as possible about the peak of the beam intensity.
Nevertheless, in a scanner in which a plurality of light beams are used, even if the beam profile of each light beam is made almost symmetrical, variations are likely to occur in the luminous energy distribution on the entire scanning plane due to displacements of the optical axes of the light beams. For example, in some cases, as shown in FIG. 17B, the peak position of the power of the first light beam L1 and the peak position of the power of the second light beam L2 are asymmetrical about a reference point (i.e., a point whose image height is zero). This results in a nonuniform luminous-energy distribution as a whole in some cases.
Therefore, a new technique for achieving a uniform luminous-energy distribution on the scanning plane in an optical scanner in which a plurality of light beams are used has been awaited.
In view of this, the present invention has been made and an object thereof is to achieve a uniform luminous-energy distribution on the scanning plane in an overfilled optical scanner in which a plurality of light beams are used.